Lecture 22- Genomic Imprinting and Dynamic Mutation

Question 1

Genomic Imprinting

Answer 1

Restriction of the expression of a gene (to either paternal or maternal allele). Expression of only one!

an ‘epigenetic phenomenon’

When an epigenitic trait occurs to only one parent

~0.1 mammalian genes show imprinting

Question 2

Epigenetics

Answer 2

an epigenetic trait is a stably heritable phenotype resulting from changes in a chromosome WITHOUT alterations in the DNA sequence. (altering instead gene expression)

Different expression from genetically identical alleles

a change in phenotype without a change in genotype

Operates at transcriptional level, eg)

• DNA (eg: to Cytosine) methylation: CPG islands placed at start of gene (promotor)
• Histone modification

Question 3

Two syndromes that occur when genetic imprinting occurs due to methylation at the region of chromosome 15 at q11
-A deletion of 15q11-q13

Answer 3

Prader-willi Syndrome (70-75% paternal copy deleted)

Angleman Syndrome (60% maternal copy deleted)

Both have overlapping clinical phenotypes

Question 4

Prader-Willi Syndrome CF

Answer 4

• failure to thrive
• neonatal hypotonia
• rapid weight gain after one year
• behavioural obesity/ short stature
• small hand and feet
• behavioural issues

Question 5

Prader-Willi Syndrome

Answer 5

Loss of PATERNAL segment of chromosome 15q11-q13

Due to imprinting, the MATERNAL normal copy is usually imprinted (and therefore silenced).

BOTH issues need to occur to get this syndrome

Question 6

Why chromosome 15q11-q13

Answer 6

recognised that distinct segments of this are required for normal development

Question 7

Anglemans Syndrome CF

Answer 7

• ‘happy children’
• mental retardation
• hypotonia

overlapping phenotypes

Question 8

Anglemans Syndrome

Answer 8

Loss(deletion) of MATERNAL segment of chromosome 15q11-q13

THe paternal segments is usually imprinted (silenced) (by methylation usually)

Question 9

Other ways (apart from having one deleted + one imprinted allele) you can get anglemans and prader-willi syndrome

Answer 9

Uniparental disomy= embryo inherits 2 copies of a locus from one parent and non from the other (PW only two imprinted maternal alleles. AM only 2 imprinted paternal alleles)

Imprinting Centre mutations: or deletions: screw up the switches of taking away methyl group

This is because 15q11-q13 contains BOTH paternally and maternally expressed genes

Question 10

Role of DNA methylation in imprinting

Answer 10

Methylation of DNA plays a huge role in controlling gene expression.
-Occurs at cytosine residues in ‘CpG islands’

CpG methylation at promoters causes ‘transcriptional silencing’

CpG methylation is transmitted through cell divisions by methytransferase enzymes

Gamete specific methylation

Question 11

CpG islands

Answer 11

> 200bp regions of DNA with lots of G+C , located mainly in PROMOTER regions

Question 12

Application and removal of imprinting

Answer 12

Female: imprinting established during oocyte maturation

male: imprinting establish prior to meiosis in primary spermatocyte

demethylation of these occur in early embryo. Primordial germ cells remain unmethylated.

This may vary between tissues and rarely parent dependent

Question 13

Methylation changes are also associated with cancer

Answer 13

Methylation of things that usually prevent cancer, ‘tumor suppressant genes’

-inherited predisposition: by inheriting a copy of gene from a parent thats methylated and that turns of a preventative cancer gene, and if the other parents gene gets damaged > cancer

• Somatic methylation changes: meethylation ‘turns on’ a tumor.
  eg) one parent has mutated BRCA gene, and the other parents normal gene gets turned off by methylation > cancer

Question 14

Inherited predisposition: Beckwith-Wiedemann syndrome

Answer 14

Pre/post natal overgrowth.

Increased tumor predisposition. (wilms tuor)

inherited methylation of genes that normally stop genes growthing (growth suppressors)

Question 15

Somatic methylation changes in cancer.

Answer 15

CCND1: gene that drives mitotic cell division

the more methylation (turning off) of this genes, the SLOWER GROWING the tumor is

The less methylation, the more genes and the more rapid the tumor growth!

Question 16

Dynamic Mutation

Answer 16

With each round of inheritance there is progressive expansion of ‘repeat sequences’, many ‘triplet repeats’. If in a coding region it will lead to repeated aa production

~20 diseases involved with repeated expansion
-Huntingtons

Symptoms in later generations occur at younger ages and are more severe = anticipation
This is bc each generation is inheriting more and more triplets!

Question 17

Huntingtons is a neuromuscular disease…

Answer 17

On chromosome 4 p arm has a huntingtons gene. Trinucleotide CAG repeats

Normal: 26 or less
Intermediate: 27-35 (nosymptoms but possibility of child inheriting 'mutant allele' if parent male)
Mutant: >36
36-39 reduced penetrance
40+ full penetrance

the more repeats… the more significant the symptoms! THey result in polyglutamine expansion, which have an effect on neuronal cells by ‘gain-of-function’.

Question 18

How are dynamic Mutations detected

Answer 18

Direct mutation analysis
Via PCR-based methods that detect alleles upto ~115 CAG repeats

Southern blotting: detects smaller repeat #

Question 19

Genotype-phentype correlation

Answer 19

The more repeats the earlier/more severe disease onset is = anticipation!!

Question 20

Anticipation

Answer 20

Juvenile-onset (usually ~ 60+ repeats) and the expanded allele is usually from the father!

Occurs more commonly with paternal transmission

Due to instability of CAG during spermatogenesis

Question 21

Risk to family

Answer 21

Most with HD have an affected parent, BUT de novo mutations can occur.

• Asymptomatic father of an affected individual may have an intermediate allele.
• Either asymptomatic parent may have a reduced penetrance allele (36-39 repeats)

AD dominant so 50% to offspring.